The present invention relates to metal-air cells having an air electrode and a liquid or paste electrolyte, and more particularly relates to metal-air cells of the type having two housing sections that must be sealed where the peripheral edges of the housing sections meet.
Considerable progress has been made in developing metal-air cells for portable electronic devices ranging from hearing aids to cellular telephones and laptop computers. Both primary and secondary cells have been developed using a variety of metal anodes and electrolyte materials, but zinc and potassium hydroxide have been particularly successful. In the case of a primary cell, the zinc and KOH are typically formed into a paste filling the volume of the cell between the air electrode and the surrounding housing walls. In secondary cells, liquid KOH fills the enclosure, surrounding a metallic zinc anode.
Leakage of electrolyte can be a problem in both primary and secondary metal-air cells. If the electrolyte has a path around the separator that lies between the cathode and the anode, for example along the housing wall, the electrolyte may leak out along the leads from the current collectors and result in the cell drying out prematurely. Some prior cells have included interior peripheral grids positioned around the wall of the cathode portion of the housing to hold down the separator and cathode. Hot melt adhesive secured the grid to the housing wall. These grids have had success in reducing leakage, but they occupy space within the housing, reducing the volume of anode paste, and require an extra gluing step during manufacture. The KOH tends to migrate through any gaps that may be left, or may develop, in the hot melt adhesive bond between the grid and the housing wall. Furthermore, there are two possible paths around the cathode/separator assembly, under the grid where it presses down on the cathode/separator assembly, and down between the grid and the housing wall.
One cell structure that has been developed attempting to solve this problem involves molding an inner peripheral wall within the anode portion of the housing, filling that portion with anode paste, and inserting the peripheral wall of the cathode portion into the gap between the anode portion walls so that the inner wall presses against the cathode assembly. Disadvantages of this approach include the need to invert the cell after assembly, and the need to form an adhesive bond within the gap.
Thus, there is a need in the art for a metal-air housing structure that resists electrolyte leakage, and allows for simple assembly.
The present invention seeks to provide an improved metal-air housing structure that protects well against electrolyte leakage, requires no hot melt adhesive, locks the cathode assembly flat in place in the housing, provides no potential leak path between a locking member and the cell housing, and allows for simple assembly, permitting the cathode portion of the housing to be filled with anode material to avoid a need for inverting the cell.
This object is accomplished in a metal-air cell constructed with an enclosure according to the present invention, comprising a lower housing section having a central member and an upstanding peripheral wall, the peripheral wall defining a top edge thereof; a generally L-shaped peripheral grid comprising an upper leg attached to the top edge of the lower housing peripheral wall, and a downwardly extending leg positioned to hold the air electrode against the central member; and an upper housing section enclosing the lower housing section and comprising an outer peripheral wall extending downwardly adjacent to and adhered to the peripheral grid and the lower housing peripheral wall. The upper leg preferably is ultrasonically welded to the top edge of the lower housing peripheral wall.
The outer peripheral wall of the described structure may be fused to the grid and the lower housing peripheral wall by a solvent bonding agent, avoiding the need for any hot melt adhesive. Furthermore, the upper leg of the L-shaped grid closes off any path between the grid and the inner wall surface of the lower housing. Any electrolyte migrating over the upper leg would have to defeat the solvent bond and pass through the ultrasonic weld to find a path between the grid and the inner wall surface. To defeat leakage under the downwardly extending leg, a sealant, such as Versamid adhesive or an asphalt sealant, may in a preferred embodiment be placed between the periphery of the air electrode and the downwardly extending leg and between the periphery of the air electrode and the central member of the lower housing.
The present invention also contemplates a corresponding method of enclosing components of a metal-air cell, comprising providing a lower housing section comprising a central member and an upstanding peripheral wall, the peripheral wall defining a top edge thereof; placing an air electrode in the lower housing section; placing a generally L-shaped peripheral grid onto the lower housing section, with an upper leg of the grid resting on the top edge of the peripheral wall, and a clamping leg extending downwardly inside the peripheral wall and pressing against the air electrode; positioning an ultrasonic horn against the upper leg of the grid and energizing the horn to weld the grid onto the top edge of the lower housing peripheral wall; filling the lower housing section with metal electrode and electrolyte material; covering the lower housing section with an upper housing section comprising an outer peripheral wall; and adhering the outer peripheral wall to the grid and to the lower housing peripheral wall.
According another aspect, the invention provides an enclosure for an electrochemical cell including an electrode, comprising a lower housing section comprising a central member and an upstanding peripheral wall, the peripheral wall defining a top edge thereof; a grid insert comprising a peripheral upper leg attached to the top edge of the lower housing peripheral wall, and a downwardly extending leg positioned to hold the electrode against the central member; and an upper housing section enclosing the lower housing section.
Other objects, features, and advantages of the present invention will be understood after reference to the following detailed description of a preferred embodiment of the invention, when taken in conjunction with the drawings and the appended claims.